Ministers warned yesterday that they would extend a freeze on the commercial planting of genetically modified (GM) crops in Britain unless biotechnology companies could guarantee their seed stocks were uncontaminated.

The companies were ordered by the Government to tighten quality controlsafter the discovery of a variety of unauthorised genetically modified oilseed rape in 14 fields in England and Scotland. Ministers have also ordered a review of their own regulatory procedures and safeguards after inspectors failed to discover the contamination for three years.

Elliot Morley, an Environment minister, expressed his anger at Aventis, the biotechnology company responsible for producing the seeds. He said the Government was considering prosecution. The Government said the farm-scale trials would not be abandoned, but suspended until guarantees had been gained.

Mr Morley said he would be prepared to delay commercial planting of GM crops beyond the moratorium that ends next year. He said the Government would not press ahead with commercial planting until it had assessed the risks.

"The real damage is to the credibility of the biotech companies in relation to quality control, and it is inevitable that people will be concerned about it if this kind of poor quality control slips through the procedures," Mr Morley said. "We have made it very clear that if we are not satisfied with the information, or there are elements of doubt, then we will be seeking to keep the moratorium [on commercial planting] in place for a longer period."

Environmental groups said the Government's programme of farm-scale trials, which began three years ago, was invalid because of doubts over whether other seed stocks were pure. The unauthorised variety of oilseed rape, which the Government was told about on 7 August, contained an extra gene that had been modified to make it resistant to at least two types of antibiotics.

Friends of the Earth called for the seed stocks of all biotechnology firms involved in the trials to be tested.